1. Field of the Invention
The present invention relates to a magnetic resonance imaging system and a magnetic resonance imaging method for imaging a subject to be examined by using an electrocardiography-synchronized imaging method (also known as “ECG-gating imaging method”) such as an FBI (Fresh Blood Imaging) method, or particularly a Flow-Spoiled FBI method.
2. Description of the Related Art
Examples of an FBI method and a Flow-Spoiled FBI (FS-FBI) method that are one of the angiographic methods used in the magnetic resonance imaging (MRI) are disclosed in Japanese Unexamined Patent Application Publication Nos. 2000-005144 and 2002-200054, respectively.
The FBI method is a non-contrast MRA method capable of virtually displaying images of the blood flow as if a contrast medium were injected. In the FBI method, a subject to be examined (hereinafter, it is simply referred to as a subject) is scanned in synchronization with the cardiac time phase of the subject, the phase being measured on the basis of ECG (electrocardiogram), PPG (photoplethysmogram), or the like, so as to capture high-speed blood flow pumped out from the heart and thus to satisfactorily depict blood vessels. Specifically, in the FBI, a 3-dimensional scanning is performed using different delay times in the diastolic period and the systolic period, the delay time being set between the reception time of an ECG trigger signal (for example, R-wave) and a data acquisition start time, and an arterial blood flow image is generated by differentiating two data components acquired in different periods from each other. As demands arise, a venous blood flow image may be obtained by differentiating the arterial blood flow image from a diastolic image (the arterial and venous blood flow images). The data differentiating process is typically configured to differentiate the blood flow images converted into the actual space from each other, but may be configured such that echo data components are differentiated from each other on a k-space having the same matrix size and the blood flow image is reconstructed on the basis of the differentiated echo data.
However, it may be difficult to distinguish the artery and the vein of the legs from each other in the MRA image captured using the FBI method, since the flow speed in the artery and vein of the legs is relatively low. Accordingly, in the imaging process of the FS-FBI method, a gradient magnetic field having a spoiler pulse (flow spoiled pulse) appended thereto is used in the systolic period. In this way, an arterial signal is suppressed in the systolic period, and the artery and the vein can be distinguished. The spoiler pulse is typically appended to the front and rear ends of a readout gradient magnetic field waveform, but may be appended to a phase encoded gradient magnetic field waveform. The arterial signal can be further suppressed by a flow-dephasing effect when a readout and encoding direction is set in a blood vessel running direction, but the readout and encoding direction may be set in other directions. The important thing is that when a phase encoding direction is set in the same direction as the blood flowing direction (blood vessel running direction), artifacts may be superimposed on the blood vessel and it is thus difficult to extract flow-voids. However, when the phase encoding direction is set in a direction different from the blood flowing direction, it is possible to attain the arterial signal suppressing effect. These imaging methods have a delay time as a parameter of imaging condition. The delay time is set by an operator. Specifically, by comparing images captured by an ECG-prep scan at a plurality of time phases, the operator calculates a time difference between the time phase at which the best image was obtained and a reference time phase (for example, the time phase at which R-wave is generated) and sets the time difference as the delay time. The ECG-prep scan is a preparation scan that executes a preparatory pulse sequence to determine synchronization timings. Additionally, in the ECG-prep scan, the subject is imaged at each of a plurality of time phases with different delay times from the reference time phase of the ECG signal.
According to the known techniques described above, the operator has to determine a proper delay time on the basis of the image obtained by the ECG-prep scan, which imposes a large burden on the operator and makes it difficult to say that the proper delay time determined by the operator is always suitable for the imaging condition.